Stéphane Ducasse 1

Stéphane Ducassestephane.ducasse@inria.frhttp://stephane.ducasse.free.fr/

Design Points - Subclassing vs Subtyping

Stéphane Ducasse --- 2010

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A little problem

How to implement a Stack?

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Stack subclass of OrderedCollection

Stack>>pop ^ self removeFirstStack>>push: anObject self addFirst: anObjectStack>>top ^ self first

Stack>>size, Stack>>includes: are free, inherited from

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Wait! but • What do we do with all the rest of the interface of

OrderedCollection?

• a Stack IS NOT an OrderedCollection!

• We cannot substitute an OrderedCollection by a Stack

• Some messages do not make sense on Stack• Stack new addLast: anObject• Stack new last

• So we have to block a lot of methods...

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Consequences...

Stack>>removeFirst self shouldNotImplement

Stack>>pop ^ super removeFirst

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The Problem• There is not a simple relationship between

Stack and OrderedCollection

• Stack interface is not an extension or subset of OrderedCollection interface

• Compare with CountingStack a subclass of Stack

• CountingStack interface is an extension of Stack interface

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Compare the two uses

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Compare the two replacements

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A Better Approach

By defining the class Stack that uses OrderedCollection

Object subclass: Stack instVarNames: ‘elements’

Stack>>push: anElement elements addFirst: anElementStack>>pop element isEmpty ifFalse: [^ element removeFirst]

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Inheritance and Polymorphism• Polymorphism works best with

conforming/substituable interfaces

• Inheritance creates families of classes with similar interfaces

• An abstract class describes an interface fulfilled by its subclasses

• Inheritance helps software reuse by making polymorphism easier

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About subtyping and subclassing• You have only extends or subclass: in

programming language.

• But they can be used to define a subclassing or subtyping relationship.

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Subtyping Inheritance • Reuse of specifications• A subclass refines superclass specifications

• A program that works with Numbers will work with Fractions.

• A program that works with Collections will work with Arrays.

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Subclassing• Inheritance for code reuse• Dictionary is a subclass of Set• Semaphore is a subclass of LinkedList• No relationship between the interfaces of the

classes

• Subclass reuses code from superclass, but has a different specification. It cannot be used everywhere its superclass is used. Usually overrides a lot of code.

• ShouldNotImplement use is a bad smell…

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Inheritance for Code Reuse • Inheritance for code reuse is good for• rapid prototyping

• getting application done quickly.

• Bad for:• easy to understand systems• reusable software• application with long life-time.

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Subtyping Essence• You reuse specification

• You should be able to substitute an instance by one of its subclasses (more or less)

• There is a relationship between the interfaces of the class and its superclass

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How to Choose?• Favor subtyping

• When you are in a hurry, do what seems easiest.

• Clean up later, make sure classes use “is-a-subtype” relationship, not just “is-implemented-like”.

• Is-a-subtype is a design decision, the compiler only enforces is-implemented-like!!!

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Quizz– Circle subclass of Point? – Poem subclass of OrderedCollection?